I've found the cause. These images use 4 colors (2 bits per pixel). That format is only available for Windows Mobile, and a regular Windows cannot display these images (unless converted first).

As a first step, I've fixed Restorator's internal image handling to support 4 color images. But it's still impossible to view them. Saving should be fine though (with the next version of Restorator). Also the error message will be more meaningful

In future, we may add a converter so that these images can be displayed nonetheless.